This application is based upon the provisional application Ser. No. 60/054,412, filed Jul. 31, 1997, which we claim as the priority date of this application.
This invention relates to fiber optic couplers and, more particularly, to 1.times.N couplers (N&gt;2) that are capable of coupling substantially equal amounts of power from an input optical fiber to three or more output optical fibers; it also relates to methods of making such couplers. "Fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by inserting the fibers into a capillary tube prior to heating and stretching the fibers, thereby resulting in the formation of an "overclad coupler". To form an overclad coupler, the fibers are inserted into a glass overclad tube, the tube is evacuated, and its midregion is heated and collapsed onto the fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling. The present invention relates to both fused fiber couplers and overclad couplers.
Fiber optic couplers that are capable of coupling light from a centrally located input optical fiber to a plurality of output optical fibers that surround the input fiber are well known. It has been known that coupling characteristics are affected by the spacing between the output fibers. Substantially equal coupling ratios require equal fiber spacing. Six output optical fibers fit around another fiber of equal diameter to form a close packed array. However, for many purposes, devices other than 1.times.6 couplers, for example 1.times.4 couplers or 1.times.8 couplers, are required.
One fiber spacing technique involves gluing each output fiber to the input fiber so that each output fiber is properly located about the circumference of the input fiber. This method is very tedious and time consuming and therefore unsuitable for commercial production.
U.S. Pat. No. 5,017,206 teaches a method of making a 1.times.N fiber optic coupler, wherein N is not equal to 6, by assembling a coupler preform comprising two concentric glass tubes having a gap therebetween. The shape and/or size of the inner tube relative to the shape and/or size of the outer tube is such that a plurality of output optical fibers can be equally spaced in the gap. If the inner surface of the outer tube and the outer surface of the inner tube are circular in cross-section, N must be greater than 6. An input optical fiber is disposed in the centrally located aperture in the inner tube. The input and output fibers extend through the midregion of the resultant coupler preform. The midregion is heated to collapse it about the fibers, and the central portion of the midregion is stretched to reduce the diameter thereof over a predetermined length. A spacer tube having an outside diameter of 205 Tm and an inside diameter of 130 Tm can be used to form a 1.times.8 coupler from commercially available optical fibers having an outside diameter of 125 Tm. The eight output fibers just fit around the spacer tube, and the input fiber fits in the spacer tube aperture. However, this embodiment of 1.times.8 coupler is difficult to implement. First, it is difficult to make the spacer tube as it is very small in diameter and has an extremely small wall thickness. Moreover, it is very difficult for a technician to insert the input fiber into the spacer tube. The tube wall is so thin that that part of the spacer tube that is grasped by a technician collapses to an elliptical cross-section. Also, the fiber can be damaged by the spacer tube as the technician is inserting the fiber into the spacer tube. The difficulty encountered in threading the input fiber into the spacer tube makes the process more labor intensive and adds to the cost of making the coupler.
When making 1.times.8 couplers by the aforementioned technique employing a spacer tube between input and output fibers, it has been conventional practice to glue the ends of the overclad tube to the fibers to improve the pull strength of those portions of the fibers that extend from the coupler. It is noted that the portion of the spacer tube that will be in the midregion of the coupler preform cannot be glued to the input fiber as the midregion is subjected to a temperature sufficient to burn the glue and ruin the coupler. Also, the glue would block evacuation of air during the tube collapse step. However, since the spacing between the input fiber and the spacer tube is extremely small, glue applied to an end of the spacer tube cannot flow between that tube and the input fiber. Therefore, an insufficient amount of glue contacts the input fiber to provide it with adequate pull strength.
To increase pull strength of the input fiber, the single spacer tube is replaced by a short length and a longer length of spacer tube. See U.S. Pat. No. 5,351,326. A drop of glue is applied to a portion of the uncoated end of the input fiber adjacent its coating, and it is inserted into the short piece of spacer tube. The glue then occupies the space between the fiber and spacer short tube. After the glue is cured, the remaining uncoated portion of the input fiber is inserted into the longer piece of spacer tube. The longer piece of spacer tube is not glued to the input fiber so that the space between them can therefore be evacuated. The length of the shorter spacer tube is sufficiently short that it is not located in the midregion of the coupler preform. The length of the longer spacer tube is sufficiently long that it extends entirely through the preform midregion. Input fiber pull strength is improved; however, it is a labor intensive process.
Another technique for making 1.times.8 couplers involves reducing the cross-sectional area of the eight output optical fibers. Methods of reducing the cross-sectional area of a fiber include etching, machining, and drawing. Such methods are difficult to control and cause the reduced diameter portion of the fiber to become fragile. Moreover, the refractive index of the central, large diameter fiber is the same as that of the cladding of the etched, small diameter fibers.